Device and method for analysing the structure of a material

ABSTRACT

The invention relates to a device that is used to analyse the structure of a material. The inventive device comprises: probe elements ( 5 ) which are used to (i) emit a wave, in the material, with emission delay laws that correspond to several simultaneous deviations and (ii) receive, on the different probe elements ( 5 ), signals from the refraction of said wave by the material; detection channels, each detection channel being connected to a probe element ( 5 ), in order to collect the refraction signals and to transmit same to data processing means ( 4 ); and delay circuits that apply a delay on each detection channel according to the reception delay laws which are predetermined and which correspond to the different deviations of the wave emitted. The invention also relates to an analysis method which can be used, in particular, on said device.

[0001] The invention relates to devices and methods for analyzing thestructure of a material.

[0002] More precisely the invention relates to devices for analyzing thestructure of a material, comprising:

[0003] a probe comprising a plurality of probe elements for the emissionof a wave, into the material, with an emission delay law correspondingto a deviation according to a direction and a focusing of this wave, andthe reception, in parallel on the various probe elements, of signalsoriginating from the refraction of this wave by the material,

[0004] a plurality of emitters, each emitter exciting a single probeelement,

[0005] a plurality of detection channels, each detection channel beingconnected to a probe element, so as to collect the refraction signalsand transmit them to data processing means,

[0006] a plurality of delay circuits, each delay circuit applying adelay, to each detection channel, according to a predetermined receptiondelay law corresponding to the deviation of the wave emitted.

[0007] Such analysis devices are already known. They are commonly usedsequentially, as is illustrated by FIG. 1. The analysis is thenperformed by firing a shot, that is to say the emission of a wave by thewhole set of probe elements according to a given deviation, for eachdesired exploration deviation. Each deviation corresponds to a distinctdelay law. The material is explored line by line by means of a movingbeam, the exploration line being shifted between each shot.

[0008] Under these conditions, the time required to construct an imagegrows with the spatial resolution demanded, with the time of travel ofthe wave in the object analyzed and with the resolution of thereconstructed image. The analysis devices of this type do not inpractice make it possible to exceed a few hundred hertz of image rate,this being hardly sufficient in numerous applications such as theinspection of sheet metal, tubes, track rails, etc.

[0009] An aim of the invention is to alleviate this drawback.

[0010] This aim is achieved, according to the invention, by an analysisdevice of the aforesaid kind in which each probe element emits the wavewith at least one other emission delay law corresponding to the emissionof the wave according to at least one other deviation, simultaneously,and the delay circuits apply delays, to each detection channel accordingto at least one other predetermined reception delay law corresponding tothe reception of the wave emitted according to the other deviation.Specifically, the emission and reception delay laws used in the deviceaccording to the invention correspond to a plurality of deviationswhich, with the devices of the prior art, would have required as manyshots as deviations, whereas the device of the invention analyzes thematerial according to various deviations, simultaneously, in a singleshot.

[0011] In preferred embodiments of the device according to theinvention, recourse is had to one and/or the other of the followingarrangements;

[0012] it comprises means of driving the delay circuits able to drivethe delay circuits so as to process each wave according to a focusingvarying as a function of time;

[0013] it comprises means of digitization of the signal collected byeach of the detection channels, with a sampling frequency lying betweentwo and five times the frequency of emission of the probe elements, andpreferably substantially equal to three times the frequency of emissionof the probe elements;

[0014] it comprises a digital interpolator with parallel outputs formultiplying the temporal resolution of the signal collected by each ofthe detection channels;

[0015] the delay circuits comprise the first delay cells and seconddelay cells in series and two summation circuits for summing the signalstransmitted, after application of the delays respectively by the firstand second delay cells;

[0016] it comprises a linear amplifier for amplifying the amplitude ofthe wave prior to its emission by the probe elements; and

[0017] the probe elements are laid out according to an arrangementchosen from among a linear arrangement, a matrix arrangement and acircular arrangement.

[0018] According to another aspect, the invention is a method ofanalyzing the structure of a material comprising:

[0019] the emission of a composite wave consisting of a plurality ofsignals emitted by a probe comprising a plurality of probe elements,with an emission delay law corresponding to a deviation according to adirection and a focusing of this wave,

[0020] the reception, in parallel, by the probe elements, of signalsoriginating from the refraction of the wave, by the material,

[0021] the transmission, by a plurality of detection channels, of thesignals received by the probe elements, to data processing means,

[0022] the application of a delay by delay circuits in parallel to eachdetection channel, according to a predetermined reception delay lawcorresponding to the deviation of the wave emitted,

[0023] characterized in that it furthermore comprises the emission of atleast one other wave, according to another deviation corresponding toanother delay law, the emission of each wave being performedsimultaneously, and in that the delay law corresponds to thesimultaneous emission of the various waves, respectively according toeach of the deviations.

[0024] In preferred modes of implementation of the method according tothe invention, recourse is had to one and/or the other of the followingarrangements:

[0025] it comprises a digitization of the signals transmitted by eachreception channel, with a sampling frequency lying between two and fivetimes the frequency of emission of the probe elements, and preferablysubstantially equal to three times the frequency of emission of theprobe elements.

[0026] it comprises in succession the steps according to which:

[0027] a first delay is applied to each signal corresponding to adetection channel,

[0028] several signals corresponding to a group of probe elements aresummed,

[0029] a second delay is applied for each group, to each signalresulting from the above summation, and

[0030] the delayed signals corresponding to the various groups aresummed;

[0031] at least two different zones of the probe, each comprising aplural of probe elements, simultaneously scan the material, according toall the deviations likewise simultaneously;

[0032] the signals received by the various probe elements are processed,in tandem with their reception, continuously, by the delay circuits; and

[0033] the signals received by the various probe elements are stored inmemory so as to be processed later by the delay circuits.

[0034] Other aspects, aims and advantages of the invention will becomeapparent on reading the detailed description which follows of one of itsembodiments.

[0035] The invention will also be better understood with the aid of thereferences to the drawings in which:

[0036]FIG. 1 diagrammatically represents the various steps implemented,with a method of the prior art, to obtain an analysis of a material,according to three different deviations;

[0037]FIG. 2 is a general schematic of an embodiment of the deviceaccording to the invention;

[0038]FIG. 3 is a schematic of a basic module of the embodiment of thedevice according to the invention represented in FIG. 2; and

[0039]FIG. 4 diagrammatically illustrates the correspondence between thelaws of delay and deviations of the wave emitted by the devicecorresponding to FIGS. 2 and 3;

[0040] An exemplary embodiment of the device according to the inventionis presented hereinbelow.

[0041] According to this example, the device according to the invention,represented in FIG. 2, comprises a probe 1, basic modules 2, asynchro-sequencer module 3 and data processing means 4. The processingmeans 4 advantageously consist of a microcomputer.

[0042] The probe 1 consists of n probe elements 5. This number n isdependent on the desired application and/or on the technological limitsof manufacture. By way of example, n=128. These n probe elements 5 aredistributed in groups 6 of four consecutive probe elements 5,distributed over the probe 1. Each group 6 is processed by a basicmodule 2.

[0043] Each probe element 5 makes it possible to emit an ultrasoundsignal into a material and to detect, in return, the signal refracted bythis material. Advantageously, each probe element 5 is of thepiezo-composite type.

[0044] A basic module 2 communicates with the next basic module 2 by wayof a summation bus 100 for summing the five deviations managed by eachbasic module 2.

[0045] The data collected by each basis module 2 are transferred to thenext basic module 2 by way of a data bus 110 consisting of a high speedserial line. The last basic module 2 transfers all of the data obtainedto the data processing means 4, via the synchro-sequencer 3, likewise byway of a data bus 110, consisting of a high speed serial line.

[0046] The synchro-sequencer module 3 has as its main function tointerface the basic modules 2 and the processing means 4 and to generatesynchronization signals on the basis of internal or external clocks, soas to transmit them to the basic modules 2 by way of a synchronizationbus 120. In particular, the interfacing between the basic modules 2 andthe processing means 4 comprises:

[0047] the storage of the data detected, in a buffer memory, via adeserializer which recovers these data, originating from the basicmodules 2, on the data bus 110;

[0048] the management of the transfers of these data to the processingmeans 4 via appropriate interfaces; and

[0049] the tailoring of the parameters of the various basic modules 2,on the basis of the data originating from the data processing means 4.

[0050] The data processing means 4 communicate with each basic module 2,via the syncho-sequencer module 3, by way of an adjustment andconfiguration bus 130.

[0051] A basic module 2 is represented in greater detail in FIG. 3. Eachbasic module 2 comprises emission and reception means 7, delay circuits8, a processor 9 and a transfer multiplexer 10.

[0052] Each basic module 2 comprises emission and reception means 7 fourtimes, that is to say once per probe element 5.

[0053] Each basic module 2 comprises delay circuits 8 five times, thatis to say once per deviation.

[0054] For each probe element 5, the emission and reception means 7comprise, for emission, an emitter memory 11, a digital/analog converter12, an analog low-pass filter 13 and an amplifier 14.

[0055] The emission and reception means 7 also comprise, for each probeelement 5, for reception, a clipper 15, an analog filter 16, an analogdigital converter 17 and a first digital interpolator 18 with paralleloutputs.

[0056] Each basic module 2 has as its function to manage the four probeselements 5 of a group 6, as well as the emission of a compound wavecorresponding to five different deviations I to V emitted by each probeelement 5 of a group 6.

[0057] The management, by each basic module 2, of the probe elements 5comprises, in particular:

[0058] the emission of ultrasounds,

[0059] the detection of ultrasound signals,

[0060] the analog/digital conversion of the signals detected, and

[0061] the digital interpolation of the signals detected, so as toincrease the temporal resolution.

[0062] For emission, emission curves are calculated in the phase ofinitialization of the device according, to the invention, with anaccuracy ten times higher than the desired resolution. Each curve isconstructed from the sum, according to emission delay laws, of thecurves necessary for each of the five predetermined deviations of theultrasound beam emitted by a given group 6. FIG. 4 illustrates, forthree deviations (0 and plus or minus 45 degrees) the form of the delaylaw, that is to say the amplitude of the signal emitted by each probeelement 5 as a function of time. These curves also take account of theapodization coefficients on emission which are intended to reduce theside lobes due to the spatial sampling of the probe 1.

[0063] These emission curves are sampled at a system frequency H. Thecorresponding sampled curves are then loaded into each emitter memory11.

[0064] Thus, for example, for a 20 MHz probe, the accuracy ofcalculation of the curves should be 0.5 nanoseconds. The samplingfrequency H may for its part be limited to around three times thefrequency of the probe 1, i.e. 60 MHz. This is sufficient for correctreconstitution of the signal and has the advantage of limiting thehardware resources and hence the cost of the device according to theinvention. By working with the lowest possible system frequency H it ispossible to obtain large delays with a minimum of hardware resources.

[0065] In the firing phase, that is to say in the phase of emission ofthe wave by the probe 1, each emitter memory 11 is read out at thesampling frequency H. The digital data corresponding to the emissioncurves recorded in each emitter memory 11 of a channel corresponding toa given probe element 5, are converted, likewise at the samplingfrequency. H, after they have been read out, into analog signals by thedigital/analog converter 12 of the same channel. The analog signalobtained is then filtered by the corresponding analog low-pass filter13, in such a way as to reject the high-frequency components due to thesampling. The filtered signal is then amplified by the correspondingamplifier 14. This amplifier 14 is a linear power amplifier making itpossible to provide the energy necessary for the excitation of a probeelement 5.

[0066] The signals emitted by the various probe elements 5 into thematerial are refracted by the latter. These refracted signals arecollected by each probe element 5. The clipper 15 protects the circuitelements 16, 17, 18 on input of the signals collected. The analog filter16 then filters the signals so as to let through only the frequencycomponents of the useful band. Finally, the signals are converted, atthe frequency H/n, by the analog/digital converter 17, with a minimumdynamic range of 14 bits. This makes it possible to obtain an inputdynamic range sufficient to avoid any analog gain control on input, andany saturation phenomenon. This dynamic range is also sufficient toprocess any of the deviations.

[0067] The first interpolator 18 performs, on the signal output by theanalog digital converter 17, an interpolation by digital filtering, soas to obtain the temporal resolution necessary for the accuracy of thedelay desired in order to perform the various deviations envisaged.Specifically, the system frequency H is, as we saw earlier, equal toaround three times the frequency of the probe 1. This would not besufficient to obtain the necessary resolution on first delay cells 19which make up the delay circuits 8 and are situated downstream of theanalog/digital converter 17, since this resolution is fixed at a tenthof the wavelength. However, the first interpolator 18 makes it possibleto solve this problem by multiplying the temporal resolution by four,without raising the system frequency H. Thus, for a 20 MHz probe and asystem frequency of 60 MHz, the resolution of the delay circuits of thedelay circuits 8 is of the order of four nanoseconds.

[0068] The delay circuits 8 comprise, for each deviation I to V, a firstdelay cell 19, a first summator 20, a second delay cell 21, a secondsummator 22, as well as a multiplier circuit 23 and processing modulesfor utilization 24, 24 a.

[0069] The delay circuits 8 manage, in particular:

[0070] the application, in parallel on the whole set of signalsdetected, delays according to reception delay laws corresponding to thevarious deviations desired,

[0071] weightings by apodization coefficients,

[0072] the digital summation in real time of the various signals delayedand weighted, so as to generate the signals corresponding to the variousdeviations desired;

[0073] the application of a gain to the delayed, weighted and summedsignals.

[0074] On output from the first interpolator 18, for each probe element5 and for each of the delay circuits 8, the first delay cell 19 managesthe delays in a group 6 of four probe elements 5.

[0075] Each first delay cell 19 is made up of a multiplexer 25, of afirst circular buffer circuit 26 and of a first divider circuit 26 a.The multiplexer 25 selects the signals emanating from the emission andreception means 7 for each probe element 5 with the resolution of atenth of the wavelength. The circular buffer circuit 26 applies a delayto each signal selected. The circular buffer circuit 26 makes itpossible to obtain delays of several wavelengths by working at thefrequency H/n. The first divider circuit 26 a carries out an apodizationin reception which reduces the side lobes due to the spatial sampling ofthe probe 1.

[0076] The first delay cell 19 is dynamically write-driven by a firstmemory circuit 27 and a second interpolator 28. The first memory circuit27 stores, in the initialization phase, the characteristic points of thedelay curve of the corresponding probe element 5. The secondinterpolator 28 regenerates the points intermediate to thesecharacteristic points.

[0077] The first delay cell 19 has a resolution of the order of a tenthof the wavelength.

[0078] The first summator 20 adds together the signals emanating fromthe first delay cells 19 of each probe element 5 of a group 6, afterthey have been put in phase by the delays applied to them by the delaycells 19.

[0079] Each second delay cell 21 comprises a second circular buffercircuit 29. Each second delay cell 21 manages the delays between thegroups 6. These delays may reach several hundred wavelengths. Each delaycell 21 is dynamically write-driven by a second memory circuit 30 and athird interpolator 31. The second memory circuit 30 stores, during thephase of initialization of the device according to the invention, thecharacteristic points of the delay curve of each group 6. The thirdinterpolator 31 regenerates the points intermediate to thecharacteristic points. The second delay cell 21 operates, like the firstdelay cell 19, at the frequency H/n.

[0080] The second delay cell 21 has a resolution of the order of a thirdof the wavelength.

[0081] The second summator 22 adds the signals of a group 6, togetherwith the signals originating from the previous group.

[0082] The transmission of the data for summation, by the secondsummator 22, between groups 6, is performed by high speed serial linkthrough a serializer 32 and a deserializer 32 a. Advantageously, theseserial links make it possible to convey bit rates of the order of 1.5Gbytes/s. Advantageously also, the deserializers are of the LVDS type(LVDS is the acronym standing for the expression “Low VoltageDifferential Signaling”) These arrangements make it possible to preserveall the bits introduced by the first 20 and second 22 summators and tothus have a perfectly linear processing chain, with an output dynamicrange of between 96 dB and 132 dB, for a probe 1 of 8 to 2043 probeelements 5.

[0083] The signal emanating from the summation by the second summator 22has a gain applied to it by the multiplier circuit 23. The multipliercircuit 23 is driven dynamically by a third memory circuit 33 and afourth interpolator 34. The third memory circuit 33 stores thecharacteristic points of again curve. This gain curve has been stored inthe third memory circuit 33 during the initialization phase. The fourthinterpolator 34 generates the intermediate points between thecharacteristic points of the gain curve that are stored in the thirdmemory circuit 33.

[0084] A fifth digital interpolator 35, with parallel output, makes itpossible to retrieve the temporal resolution of the tenth of awavelength without raising the system frequency H. This resolution issufficient to permit all the conventional processing performed byconventional apparatus for inspection and imaging by ultrasound. Thedata output by the fifth interpolator 35 are processed by the processingmodules for utilization 24, 24 a. These processing modules forutilization 24, 24 a are advantageously respectively of the “echodisplay” (A-SCAN) type and “measurement of amplitudes and of distancesin a selection window” (GATES) type. These processing modules forutilization 24, 24 a are entirely conventional within the field ofnondestructive inspection and imaging by ultrasound.

[0085] The signal resulting from the processing by the processingmodules for utilization 24, 24 a is dispatched by the processor 9 andthe transfer multiplexer 10, to the next basic module 2 or to the dataprocessing means 4, via a high speed serial link of LVDS type.

[0086] In the above described exemplary device according to theinvention, the reception delay laws are applied, on the one hand, at thelevel of the first delay cells 19 and, on the other hand, at the levelof the second delay cells 21. This makes it possible to have:

[0087] first delay cells 19 that can apply delays equal to, for example,forty times the length of the wave emitted into the material, with arelatively good resolution (for example of the order of a tenth of thewavelength); and

[0088] second delay cells 21 that can apply delays equal to, forexample, one hundred and sixty times the length of the wave emitted intothe material, with a relatively low resolution (for example of the orderof a third of the wavelength).

[0089] The device according to the invention allows the summation ofsignals that are out of phase by 0 to several hundred wavelengths, witha resolution of the order of a tenth of the wavelength. By virtue of thedevice according to the invention, deviations of the ultrasound beamemitted by the probe 1 of from 0 degrees to plus or minus 90 degrees areobtained, each deviation being, able to take any value whatsoever inthis range independently of the values of the other deviations. Thedevice according to the invention permitting moreover the frequencycoding of the signals emitted for each deviation, the discrimination ofthe signs received is thereby greatly improved.

[0090] By way of example, for a 20 MH probe and hence a period of 50nanoseconds, the out-of-phase span can vary from 0 to around 10microseconds, with a resolution of five nanoseconds.

[0091] The delays are applied with an accuracy that is sufficient for itto be possible to abandon oversampling points so as to return to thevolume and to the frequency of processing of the signals, beforeinterpolation by the first interpolator 18.

[0092] The exemplary embodiment of the device presented above allowsfully real time operation. The acquisition and processing rate dependsonly on the time of travel in the material analyzed, as in theconventional one-channel inspection systems, thereby permitting rates ofseveral kilohertz in the majority of applications. This device comprisesno acquisition memory, hence enabling substantial and unlimiteddurations of probing, without any cost overhead in hardware componentsfor the analysis device. When the inspection conditions do notnecessitate rates of several kilohertz, the device according to theinvention can advantageously be associated with a head-end multiplexerwhich makes it possible to substantially reduce the overall cost of theanalysis device. Moreover, the use of digital delay circuits allowsangular deflections markedly greater than those obtained with analogcircuits. Specifically, the accuracy of digital circuits does not dependon the absolute value of the delay.

[0093] The device according to the invention operates in a circularmanner, that is to say emission and reception can be started and stoppedon any probe element 5 whatsoever.

[0094] The above described device according to the invention allows theimplementation of several methods likewise in accordance with thepresent invention.

[0095] According to a first exemplary method according to the invention,a composite wave made up of several signals emitted by the whole set ofprobe elements 5 of the probe 1 is emitted with emission delay lawsallowing the simultaneous emission of the wave according to severaldeviations. The delay circuits 8 then process the signals correspondingto the wave emitted, which signals are refracted by the material andcollected by the probe elements 5, while applying, to these signals,reception delay laws corresponding to the various deviations.

[0096] According to a second exemplary method according to theinvention, a composite wave made up of several signals emittedsimultaneously by at least two different zones of the probe 1 eachcomprising several probe elements 5 is emitted. Each of these zones thensimultaneously scans the material to be analyzed, emitting the waveaccording to several deviations. We then process the signals refractedby the material and collected by the probe elements 5, zone by zone,applying reception delay laws to them corresponding to the variousdeviations of the wave emitted.

[0097] This second exemplary method is particularly advantageous.Specifically, for example by dividing the surface of the material to beanalyzed into four regions and analyzing each of these regions with adifferent zone of the probe 1, the scanning rate is divided by four. If,with each of the zones, the wave is emitted according to fivedeviations, then a factor of twenty is gained in the rate of scanning ofthe material to be analyzed with respect to what could be obtained withthe methods and devices of the prior art.

[0098] The device and the method according to the invention apply, notexclusively, to all inspections by ultrasound, namely, in the field ofnon destructive inspection to portable apparatus and to automaticsystems, and in the medical field, to diagnostic apparatus.

[0099] The device and the method according to the invention apply alsoto all systems which use a sensor consisting of a plurality ofindependent elements, whatever physical phenomenon is utilized: eddycurrents, infrasound, electromagnetic waves, etc.

[0100] The device and the method according to the invention apply alsoto all multi-element sensors, be they linear, matrix-like or circular.

[0101] Numerous variants of the device and the method according to theinvention may be envisaged with the embodiments and modes ofimplementation described above, without departing from the scope of theinvention.

[0102] Thus, instead of the signals collected by the probe elements 5being processed in real time, it is possible to provide for a buffermemory upstream of the delay circuits 8, so as to record these signalswith a view to subsequent processing by these delay circuits 8.

[0103] Likewise, described hereinabove is a device comprising groups 6of four probe elements 5, emitting according to five deviations, but thenumbers of groups 6, of probe elements 5 per group 6 and of deviationsis variable and depends on the applications and/or on the performance ofthe hardware components used for the manufacture of the device accordingto the invention.

1. A device for analyzing the structure of a material, comprising: aprobe comprising a plurality of probe elements (5) for the emission of awave, into the material, with an emission delay law corresponding to adeviation according to a direction and a focusing of this wave, and thereception, in parallel on the various probe elements (5), of signalsoriginating from the refraction of this wave by the material, aplurality of emitters (7), each emitter (7) exciting a single probeelement (5), a plurality of detection channels, each detection channelbeing connected to a probe element (5), so as to collect the refractionsignals and transmit them to data processing means (4), a plurality ofdelay circuits (8), each delay circuit (8) applying a delay, to eachdetection channel, according to a predetermined reception delay lawcorresponding to the deviation of the wave emitted, characterized inthat, each probe element (5) emits the wave with at least one otheremission delay law corresponding to the emission of the wave accordingto at least one other deviation, simultaneously, and that the delaycircuits apply delays to each detection channel according to at leastone other predetermined reception delay law corresponding to thereception of the wave emitted according to the other deviation.
 2. Thedevice as claimed in claim 1, comprising means of driving the delaycircuits (8) able to drive the delay circuits (8) so as to process eachwave according to a focusing varying as a function of time.
 3. Thedevice as claimed in claim 1, comprising means of digitization (17) ofthe signal collected by each of the detection channels, with a samplingfrequency lying between two and five times the frequency of emission ofthe probe elements (5), and preferably substantially equal to threetimes the frequency of emission of the probe elements (5).
 4. The deviceas claimed in claim 1, comprising a digital interpolator (18) withparallel outputs for multiplying the temporal resolution of the signalcollected by each of the detection channels.
 5. The device as claimed inclaim 1, in which the delay circuits (8) comprise first delay cells (19)and second delay cells (21) in series and two summation circuits (2 0,22) for summing the signals transmitted, after application of the delaysrespectively by the first (19) and second (21) delay cells.
 6. Thedevice as claimed in claim 1, comprising a linear amplifier foramplifying the amplitude of the wave prior to its emission by the probeelements (5).
 7. The device as claimed in claim 1, in which the probeelements (5) are laid out according to an arrangement chosen from amonga linear arrangement, a matrix arrangement and a circular arrangement.8. A method of analyzing the structure of a material comprising, theemission of a composite wave consisting of a plurality of signalsemitted by a probe comprising a plurality of probe elements (5), with anemission delay law corresponding to a deviation according to a directionand a focusing of this wave, the reception, in parallel, by the probeelements (5), of signals originating from the refraction of the wave, bythe material, the transmission, by a plurality of detection channels, ofthe signals received by the probe elements (5), to data processing means(4), the application of a delay by delay circuits (8) in parallel toeach detection channel, according to a predetermined reception delay lawcorresponding to the deviation of the wave emitted, characterized inthat it furthermore comprises the emission of at least one other wave,according to another deviation corresponding to another delay law, theemission of each wave being performed simultaneously.
 9. The method asclaimed in claim 8, comprising a digitization of the signals transmittedby each reception channel, with a sampling frequency lying between twoand five times the frequency of emission of the probe elements (5), andpreferably substantially equal to three times the frequency of emissionof the probe elements (5).
 10. The method as claimed in claim 8, inwhich, all the deviations are processed in parallel and for eachdeviation, in succession, a first delay is applied to each signalcorresponding to a detection channel, several signals corresponding to agroup (6) of probe elements (5) are summed, a second delay is appliedfor each group (6), to each signal resulting from the above summation,and the delayed signals corresponding to the various groups (6) aresummed.
 11. The method as claimed in claim 8, in which at least twodifferent zones of the probe (1), each comprising a plurality of probeelements (5), simultaneously scan the material, according to all thedeviations likewise simultaneously.
 12. The method as claimed in claim8, in which the signals received by the various probe elements (5) areprocessed, in tandem with their reception, continuously, by the delaycircuits (8).
 13. The method as claimed in claim 8, in which the signalsreceived by the various probe elements (5) are stored in memory so as tobe processed later by the delay circuits (8).